Consideringthis voltage threshold and adding a reasonable voltage drop for theinductor L 1 and the diode D 2, IC1 must turn onwhen the IN voltage drops below 11.5V. However, to optimize efficiency,IC1 must not run when the battery voltage is at its normal level (IN 12V).

These low input voltages occur during a warm crank (when the battery voltage can drop as low as 6V) or a cold crank (when the battery voltage can drop as low as 3V).

The power supply assures a stable 8V supply for common subsystems such as a CD driver, LCDs, and a radio module in modern infotainment systems.

To avoid disturbance in the AM and FM bands, the switching power supply runs at a fixed frequency of 2MHz, enabling an ideal solution for radio systems.

Additionally, a boost converter can operate off the 3.3V to regulate 5V (e.g., for the CAN transceiver) or other higher voltages (Case 2).

The design presented here meets this challenge by operating the switching converters above the AM band, i.e.

MHz (the high end of the Medium Wave band).

Operating switching converters at high frequencies also reduces the size and cost of external passive components.

It incorporates a 4.5V to 40V step-up controller (IC1) and a 36V step-down controller (IC2), with additional circuitry for proper operation.

Both ICs are synchronized with an external 2MHz square-wave logic signal, provided by a microcontroller or dedicated IC.

This approach allows great flexibility in choosing the optimal switching frequency for the power supply.

When the battery voltage decreases during cold crank, IC1 is enabled and boosts the voltage on the OUTA node.

Because of the robustness of both ICs, the entire design can survive up to a 40V automotive load dump.

The system has been set up and tested to provide 20W of power (8V at 2.5A) on its main output (OUTB), although the external components can be modified to reach even higher output power.

See discussion about optimizing external components for both IC1 and IC2 below.).

Note first that the data sheet for IC2 recommends low-side resistors of less than 100k.

Choosing a 51k low-side resistor-divider for R 22, the high-side resistor-divider must be selected using the Equation 1.

Since we are synchronizing IC2s switching frequency with an external 2MHz signal, we must choose an R 16 internal oscillator resistance that imposes an internal switching frequency less than 1.8MHz.

To make IC2 switch at a 2MHz fixed frequency, it is necessary to avoid the dropout condition.

This IC avoids dropout until the turn-off time (t OFF ) is higher than 100ns (typ).

This implies that the system can never go over a maximum duty cycle of.

Thus, during load-dump peak, IC2 should operate with a duty cycle of.

Accomplishing this, however, requires more printed-circuit board (PCB) area and degrades the load regulation.

As an acceptable compromise, choose the inductor value so you obtain an LIR (the ratio of the inductor peak-to-peak AC current to the DC average current) equal or lower than 0.3.

Consider the following equations based on Figure 3.

Leaving margin for the tolerance onthe inductor, the sense resistor has been sized to have a voltage dropof 60 of the current-limit threshold when the inductor current reachesits peak value (I PEAK ).

For this design we turned offthe device when the input voltage was lower than 5V; we assumed thatthere would be a higher voltage during the cold-crank phase.

Havingselected a 100k resistor for R 5, Equation 16 is used to choose the R 4.

Consideringthis voltage threshold and adding a reasonable voltage drop for theinductor L 1 and the diode D 2, IC1 must turn onwhen the IN voltage drops below 11.5V.

However, to optimize efficiency,IC1 must not run when the battery voltage is at its normal level (IN 12V).

Smps Disable Output Protection